The present invention relates generally to extracorporeal fluid circuits and, more particularly, to a compact membrane oxygenator and combined reservoir/heat exchanger used alone or in conjunction to reduce the prime volume of an extracorporeal blood circuit.
Cardiopulmonary bypass (CPB) surgery requires a perfusion system, or extracorporeal oxygenation circuit, to maintain an adequate supply of oxygen in the patient""s blood during the surgery. A venous return cannula inserted in one of the veins leading directly to the heart receives the xe2x80x9cusedxe2x80x9d blood for rejuvenation through the perfusion system. The blood flows out of the patient into an extracorporeal fluid circuit having a conduit (typically a transparent flexible tube) to a venous reservoir that may also receive fluid from cardiotomy suckers. Commonly, one or more suckers extracts excess fluid from the chest cavity during the operation and diverts the fluid, which may contain bone chips or other particulates, into the top of the reservoir.
Typically, a centrifugal or roller pump impels blood, for example, from the venous/cardiotomy reservoir through a blood oxygenator and back to the patient. The pump assumes the pumping task of the heart and perfuses the patient""s circulatory system. The oxygenator directs a flow of blood across a semi-permeable membrane or a plurality of semi-permeable fibers to transfer oxygen to and carbon dioxide from the blood. The oxygenator often incorporates a heat exchange system to regulate the extracorporeal blood temperature, termed a xe2x80x9cclosedxe2x80x9d system. Before reaching the patient, the blood may pass through a temperature control monitoring system and along a conduit through an arterial filter and bubble detector, before reaching an arterial cannula positioned in a main artery of the patient.
The various components such as the reservoir, oxygenator and arterial filter require a minimum volume of blood to begin circulation. All of the components taken together require a xe2x80x9cprimexe2x80x9d volume of blood defined as that volume of blood outside the patient, or extracorporeal. The term xe2x80x9cprime volumexe2x80x9d can also be used to specify the volumetric capacity of each extracorporeal component in the system.
There are number of performance measurements for oxygenators. Important considerations include gas transfer capabilities, priming volume, blood compatibility, sterility, assembly, and maintenance. Effective oxygenators provided sufficient gas transfer with a minimum pressure drop and prime volume. In addition, the flow capacity through the oxygenator must be sufficient for the particular patient. Often, there is a trade-off in one or more of these performance characteristics to obtain a low priming volume or high flow rate, for example.
The need for a large prime volume in an extracorporeal fluid circuit is contrary to the best interest of the patient who is undergoing the surgery and is in need of the maximum possible amount of fully oxygenated blood. This is especially true of smaller adults, children, and pediatric or infant patients. Therefore, a significant amount of research and development has been directed toward reducing the prime volume within CPB systems. One area in which such a reduction of volume can be attained is to reduce the volume of the individual components, such as the reservoir, or blood oxygenator. There are limits to how small these components can be made, however, such as a need for adequate oxygen transfer to the blood, which depends in part on a sufficient blood/membrane interface area.
Much of the development in recent years has been toward reducing the prime volume of oxygenators while maintaining adequate flow rate and gas transfer capabilities. Unfortunately, this is not an easily attainable goal, and many of the smallest prime volume oxygenators have such a reduced flow rate that they are only useful for neonatal or infant patients, or exhibit some other performance disadvantage. Conversely, many oxygenators which otherwise have adequate performance, require a higher priming volume. For example, most of the most widely used commercial membrane oxygenators on the market for adult patients have priming volumes of between 0.3 and 0.6 liters. Given the limited supply of the patient""s blood, any decrease in priming volume in the oxygenator or other components of the extracorporeal circuit greatly enhances the chances for a positive surgery and rapid recovery.
In spite of ongoing advances in extracorporeal circuit technology, there exists an ever-present need for a reduced prime CPB system.
The present invention provides an improved low prime extracorporeal system including a low prime oxygenator and a low prime combined heat exchanger/reservoir. The dimensions of the oxygenator are optimized so that, in conjunction with a particularly preferred hollow fiber architecture, a prime reduction from currently available models as well as top performance results. Two sizes of oxygenator are disclosed which have the capacity to fulfill the needs of all ranges of patient weights, from the smallest neonatal baby to large adults. The oxygenators share certain preferred dimensions and elements, and essentially just differ in height. The combined heat exchanger/reservoir makes use of a single-pass guided heat exchanger configuration that decouples the heat exchange efficiency from the reservoir blood level.
In one embodiment, the low prime oxygenator, comprises a rigid housing defining an annular oxygenation chamber having a first axial end and a second axial end. A plurality of elongated, hollow, semi-permeable fibers are arranged in an annular bundle in the oxygenation chamber and secured at both axial ends with a potting compound. The bundle substantially fills the oxygenation chamber with the fibers arranged to provide blood flow spaces therebetween, and the opposed ends of the fibers are open to a gas header space formed in the housing outside of the oxygenation chamber. A central blood inlet port is provided in communication with a blood distribution space adjacent one axial end of the oxygenation chamber. A plurality of blood inlets in the housing are formed around the annular oxygenation chamber in communication with the blood distribution space, while a plurality of blood outlets in the housing are formed around the annular oxygenation chamber on the axial end opposite the blood inlets. In an embodiment of the oxygenator suitable for adults, the oxygenator has a prime volume of between 130 and 180 ml and a ratio of oxygen transfer rate to prime volume of at least about 0.34 lpm/min, at a flow rate of about 7 lpm. In an embodiment of the oxygenator suitable for neonatal/infants, the oxygenator has a prime volume of between about 56 ml and 80 ml and an oxygen transfer rate of about 62.5 ml/min/lpm at a flow rate of about 2 lpm.
The blood oxygenator of the present invention desirably has a simplified construction with a rigid housing consisting essentially of five parts, including: an inner core having a radial bottom wall and a cylindrical wall, an outer cylindrical wall concentric about the inner core cylindrical wall defining an annular oxygenation chamber therebetween having a first axial end and a second axial end, a pair of end caps connected to opposite ends of the outer cylindrical wall, and a blood inlet cap secured to the inner core. The inlet cap has a central blood inlet port in communication with a blood distribution space adjacent one axial end of the oxygenation chamber and formed between the inlet cap and the inner core bottom wall. A plurality of blood inlets in the inner core are formed around the blood distribution space in communication with the annular oxygenation chamber. The oxygenator includes a plurality of elongated, hollow, semi-permeable fibers arranged in an annular bundle in the oxygenation chamber and secured at both axial ends with a potting compound. The opposed ends of the fibers are open to a gas header space formed within the end caps outside of the oxygenation chamber. The bundle substantially fills the oxygenation chamber with the fibers having blood flow spaces therebetween. A plurality of blood outlets in the outer cylindrical wall are formed around the annular oxygenation chamber on the axial end opposite the blood inlets causing generally axial flow of blood through the oxygenation chamber and between the hollow fibers. The five parts of the oxygenator are either snap-fit together with O-ring seals, or are bonded with adhesive or UV welds.
The present invention also embodies an extracorporeal system, comprising a combined heat exchanger/blood reservoir and a hollow fiber oxygenator. The reservoir has heat exchange elements located in a separate heat exchange chamber and a blood outlet. The oxygenator includes a blood inlet connected to the blood outlet of the heat exchanger/blood reservoir, and a rigid housing defining an annular oxygenation chamber having a cross-sectional area normal to its axis of between about 24 and 28 square centimeters. The oxygenation chamber has a first axial end and a second axial end, and the housing includes a central blood inlet port in communication with a blood distribution space adjacent one axial end of the oxygenation chamber. A plurality of blood inlets in the housing are formed around the annular oxygenation chamber in communication with the blood distribution space, while a plurality of blood outlets in the housing are formed around the annular oxygenation chamber on the axial end opposite the blood inlets. Finally, a plurality of elongated, hollow, semi-permeable fibers arranged in an annular bundle in the oxygenation chamber and secured at both axial ends with a potting compound. The opposed ends of the fibers are open to a gas header space formed in the housing outside of the oxygenation chamber. The fibers having an aggregate volume that is between 0.5 and 0.6 of the volume in the oxygenation chamber between the potting compound at both axial ends.
A combined heat exchanger/blood reservoir, including a housing topped by a lid together defining a reservoir chamber within, a venous blood inlet in the lid, a heat exchanger within the chamber including a plurality of heat exchange elements, and a blood outlet in a lower portion of the reservoir chamber. The heat exchange chamber is defined by guides closely surrounding the heat exchange elements and extending downward from a location at an upper portion of the reservoir chamber. The heat exchange chamber has an upper inlet open to the venous blood inlet and a lower outlet open to the reservoir chamber so that blood from the venous blood inlet must flow through the heat exchange chamber before reaching the reservoir chamber. Preferably, the guides are concentric tubes defining an annular heat exchange chamber terminating at an elevation about xc2xc of the distance from the bottom of the reservoir chamber.
Further objects and advantages of the present invention shall become apparent to those skilled in the art upon reading and understanding the following detailed description of a presently preferred embodiment of the invention.